1. Field of the Invention
The present invention relates to a process for the highly selective preparation of mononitrotoluenes, which is free from waste acid and utilizes the heat of reaction.
Mononitrotoluenes are important intermediates for the production of plastics, dyes and auxiliaries.
2. Description of the Related Art
Mononitrotoluenes are prepared industrially by isothermic nitration of toluene at low temperatures (20.degree. to 60.degree. C.). Large amounts of highly contaminated waste acid arise in this, which must be disposed of or further processed expensively. A disadvantage of this process is that a considerable heat of reaction must be removed and this energy arises at a low temperature level so that it cannot be utilized.
A further disadvantage is that additional energy must be employed to concentrate the cold spent acid. Additionally, the separation of organic and inorganic phases after the nitration poses difficulties. Residual organic material must be removed from the spent acid by extraction with toluene. A known phenomenon in this is the black colouration of the spent acid ("black spend acid", U.S. Pat. No. 4,650,912), which leads to problems in the acid concentration.
In order to avoid the production of waste acid, processes must be sought which contain an integrated sulphuric acid concentration with utilization of the heat of reaction. This necessitates a circulating acid in which bi-products may not accumulate and high reaction temperatures in order to be able to concentrate the sulphuric acid industrially in an inexpensive manner.
For benzene, the adiabatic mononitration is described in a number of patents (U.S. Pat. Nos. 2,256,999, 4,021,498, 4,091,042, 4,973,770, EP 436443). The abovementioned energetic disadvantages in the isothermal toluene nitration do not apply to the adiabatic benzene nitration, since the heat of reaction thereof arises at a higher temperature level (temperature of the spent acid at the end of the reaction, for example &gt;100.degree. C.) and can be used to concentrate the acid. Although the extension of this proceeds the mononitration of toluene as mentioned in the abovementioned applications, it has never been described in an example. In contrast, the literature discloses that mononitrotoluenes are prepared isothermally at low temperatures (approximately 20.degree.-60.degree. C.) in order to avoid the intensified production and relatively high temperatures of dinitrotoluenes and oxidation products, such as nitrocresols and benzoic acids (Houben-Weyl, Volume X/1, p. 156, Thieme Verlag, Stuttgart 1971; Kirk-Othmer, Vol. 9, p. 395, Interscience, New York 1952). It is further known that the isothermal dinitration is carried out already in the temperature range of approximately 65.degree.-85.degree. C. It therefore had to be assumed that the adiabatic nitration of aromatic compounds other then benzene according to the above patents has a purely speculative character. In particular, the temperature quoted of 100.degree. C. for the nitration of benzene in the example of U.S. Pat. No. 4,973,770 appeared not to be applicable to toluene.
In the process described in U.S. Pat. No. 4,973,770, attention is further focused on a single atomization and fine distribution of the benzene, by the aid of which the entire course of the reaction is to be controlled. In order to maintain the fine distribution for as long as possible, the coalescence of the atomized particles with each other and on the wall must be avoided. To avoid wall contact, a reactor having a large diameter relative to the nozzle diameter is used; as a result strong back-mixing is produced as in an ideally mixed stirred tank: according to exemplary embodiment of U.S. Pat. No. 4,973,770 in combination with FIG. 1, benzene is introduced into a reactor having a diameter of 75 mm through a spray nozzle having a diameter of 0.5 mm. With a total length of the reactor of 430 mm, the mixed acid is added at a distance of 150 mm from the nozzle, where the energy of the nozzle jet is already substantially consumed for the back-mixing. The mixing of the added mixed acid with the injected benzene therefore takes place at a lower energy level and with a lower intensity.
The claim of EP 436 443, according to which mixed acids containing nitronium ions are employed having molar compositions which are to be taken from FIG. 6 in EP 436 443 is also outside that which is familiar to those skilled in the art. Thus the point D emphasized in FIG. 6, in combination with p. 9, lines 10-12 of EP 436 443, indicates an acid of the composition 72.02 mol % H.sub.2 SO.sub.4, 2.99 mol % HNO.sub.3 and 24.99 mol % H.sub.2 O which is equivalent to 91.71% by weight of H.sub.2 SO.sub.4, 2.45% by weight of HNO.sub.3 and 5.84% by weight of H.sub.2 O. Such a strong acid is not very suitable for the toluene nitration, in particular for the mononitration. Furthermore, circulation of the waste acid formed in this process is not industrially useful, since the necessary concentration to far above 90% by weight is too complex. This process is therefore not economically expedient and cannot provide those skilled in the art with information on achieving the present object.